In recent years, non-contact power supply technology transmitting power without contacting and direct electrical connection has been developed.
FIG. 11 is a block diagram of conventional resonance-type non-contact power supply system 500 disclosed in PTL 1. Power supply system 500 charges a secondary battery using non-contact power supply technology. The system includes power-supply equipment 101 and mobile-object-side equipment 123. Power-supply-side equipment 101 includes primary resonant coil 113 that has power supplied from high-frequency power supply 110. Mobile-object-side equipment 123 includes secondary resonant coil 121 that receives power from primary resonant coil 113, voltage sensor 127 that detects the voltage of battery 125, and vehicle-side controller 129 that senses the state of charge according the detection signal.
Vehicle-side controller 129 estimates the impedance value of battery 125 and determines that a foreign matter exists between primary resonant coil 113 and secondary resonant coil 121 if the absolute value of the difference between the estimated impedance value and the present impedance value of battery 125 is larger than a threshold. This system can detect the foreign matter that adversely affects power transmission without a dedicated sensor.